Brillouin optical fiber sensor for cryogenic thermometry

摘要

Supraconductive installations are now commonly used in large facilities, such as power plants and particle accelerators. This requires a permanent temperature control at very low temperature, but cryogenic temperature measurements in the 1-77K range requires expensive calibrated temperature probes. We report here the possibility to use stimulated Brillouin scattering in optical fibres for temperature sensing down to lK. Such a technique offers the additional advantage to make possible distributed measurement, so that very large structures and systems can be controlled using a single fiber and a single analysing instrument. In addition only one bypass for the fiber is required as input to the cryogenic vessel, that is definitely a key advantage for the design and the energy loss. Brillouin scattering in optical fibers has never been investigated so far at temperature below 77K (nitrogen boiling point). This absence of interest probably results from the constant decrease of scattering efficiency that was observed while cooling the fiber down to 77K. Our measurements show the unexpected feature that scattering efficiency is significantly raised below 50K and is even much better than observed at room temperature. The relevance and the feasibility of the technique is demonstrated in real scale on the supraconductive magnets for the future world largest particle accelerator, namely the large hadron collider (LHC) at CERN Laboratory in Geneva.